Acoustic waves are detected by currently available glass break detectors and intrusion detectors. Conventional glass break detectors of the acoustic wave type are designed to eliminate false alarms by detecting the presence of high and low frequency components of acoustic wave signals produced by breaking glass. U.S. Pat. No. 4,091,660 of Yanagi describes a glass break detector that enables an alarm whenever the detector detects the presence of acoustic wave signals of below 50 kHz and above 100 kHz.
Other devices are designed to detect the presence of different acoustic wave frequency components at different times. The premise underlying the operation of the detector described in U.S. Pat. No. 4,668,941 of Davenport et al. is that breaking glass produces an initial low frequency component centered around 350 Hz and a subsequent high frequency component centered around 6.5 kHz. The 6.5 kHz component is indicative of a tinkling sound of glass breaking as it falls to the ground and shatters. However, U.S. Pat. No. 4,837,558 of et al. points out the shortcomings of the Davenport et al. detector by emphasizing that breaking glass may not produce the tinkling sound, particularly if the glass pane or window is situated above a carpeted surface.
Conventional intrusion detectors detect an infrasonic pressure wave that accompanies the opening of a door or window. Such pressure waves may be detected by a sensitive microphone or other acoustic transducer having a frequency response in the region of 1-10 Hz. U.S. Pat. No. 4,853,677 of Yarbrough et al. describes such a device that also included a glass break detector circuit that is coupled to a microphone. An event causing either a high frequency or a low frequency signal having a predetermined frequency spectrum triggers an alarm. U.S. Pat. No. 4,991,145 of Goldstein et al. describes an acoustic intrusion detector that detects the opening of a door or window by responding to negative-going air pressure waves.
U.S. Pat. No. 5,192,931 of Smith et al. describes a glass break detector that includes an acoustic transducer having a wide band frequency response and coupled to a dual channel filter and signal processing circuit. A low frequency channel filter detects an initial positive compression acoustic wave caused by an inward motion of a glass window, and a high frequency channel filter detects an acoustic wave frequency spectrum that is characteristic of breaking glass. A coincidence logic circuit enables an alarm whenever the positive low frequency acoustic wave is detected during a predetermined time interval that begins with a high frequency event (i.e., a loud sound) generated by the breaking glass. The presence of a requisite high frequency spectrum of acoustic waves following the initial positive low frequency produced by the inward motion of the glass window triggers the alarm. The detector includes false alarm circuitry that inhibits the alarm upon the detection of an initial negative compression wave followed by high frequency sounds.
Although they take advantage of certain acoustic wave signal characteristics of breaking glass, the previously described glass breakage and intrusion detectors do not always inhibit false alarms that may be produced by events having frequency characteristics similar to those produced by breaking glass.